The Collaborative International Dictionary of English v.0.48:

Buckthorn \Buck"thorn`\, n. (Bot.)
   A genus (Rhamnus) of shrubs or trees. The shorter branches
   of some species terminate in long spines or thorns. See
   Rhamnus.
   [1913 Webster]

   Sea buckthorn, a plant of the genus Hippopha["e].
      [1913 Webster]

WordNet (r) 3.0 (2006):

buckthorn
    n 1: a shrub or shrubby tree of the genus Rhamnus; fruits are
         source of yellow dyes or pigments
    2: any shrub or small tree of the genus Bumelia
    3: an Old World plantain with long narrow ribbed leaves widely
       established in temperate regions [syn: English plantain,
       narrow-leaved plantain, ribgrass, ribwort, ripple-
       grass, buckthorn, Plantago lanceolata]